An optical isolator is a device used to create an optical light valve; i.e. allowing light to travel in one direction, but not in the reverse. In practical terms, isolators are quite useful, and indeed often necessary, in controlling certain types of light and their functional applications. Optical isolators are often employed along with lasers in their various operations and applications because the isolators prevent undesired reflection of laser light back into the laser (i.e., "feedback"). Feedback is undesirable because it can cause destabilization, parasitic oscillations, optical damage and other significant problems if left uncontrolled. For example, isolators are used in laser-based optical communications devices that require stable oscillation of the laser, including laser diodes.
In most circumstances, an optical isolator will incorporate a structure referred to as a "Faraday rotator" to manage and control the behavior of light--particularly laser light--in a desired fashion. A Faraday rotator's operation and use are based upon the "Faraday effect," a term that refers to the rotation of the plane of polarization of light propagating through a medium in the presence of an externally applied magnetic field. The angle ".theta." to which the polarized light is rotated is proportional to the distance "d" that the light travels through the medium, to the magnetic field strength &lt;H&gt; averaged along the optical axis, and to a characteristic of the medium referred to as the Verdet constant ("V"; degrees per Oersted-centimeter) of the medium; all according to the relationship: .theta.=V &lt;H&gt; d.
The general structure of an optical isolator consists of a Faraday rotator flanked by two polarizers. The polarizers are usually oriented at 45.degree. with respect to one another, and the Faraday rotator is typically selected to give a 45.degree. rotation of its own. In turn, the Faraday rotator generally consists of a Faraday medium material (often referred to as an "optical rod" in visible and near-IR applications) placed within a structure of magnets that are intended to produce the desired magnetic field. In many conventional rotators, the magnets are generally cylindrical with a circular aperture that contains the optical rod. In one common embodiment, the cylindrical magnet assembly is formed of a longer central magnet flanked by two smaller magnets in the opposite pole-to-pole relationship commonly used to try to produce a high magnetic field along the Faraday medium.
Because of the relationship between rotation of the plane of polarization, the Verdet constant, the magnetic field strength, and the rod length, the main techniques for increasing or otherwise controlling the angle of rotation consist of selecting a material with a higher Verdet constant, increasing the magnetic field strength, or increasing the length of the optical rod. The Verdet constant is, however, fixed for any given material and any specific wavelength, and thus if the material selected for the optical rod is otherwise satisfactory (or for some applications, necessary), the only options are to increase the magnetic field or the length of the rod. Additionally, because the Verdet constant is inversely proportional to the square of the wavelength of the light being rotated, the length of optical rod required for a given rotation will increase as the wavelength increases. Furthermore, increasing the length (d) of the optical rod generally tends to increase absorption losses, the undesirable effect of self-focusing, cost, and limits the types of materials that can be used.
Increasing the magnetic field thus represents an attractive technique for obtaining the usually desired 45.degree. angle of rotation from a Faraday rotator. Suggested techniques have included using larger numbers of smaller magnets. Unfortunately, obtaining higher magnetic field strength using more, but smaller magnets presents a number of practical problems, including the difficulties of assembling and packaging large numbers of small, high strength magnets in the opposing pole relationship typical of Faraday rotators.
As an additional problem, many of the common cylindrical magnet rotators suffer from uniformity problems. Specifically, the magnetic fields &lt;H&gt; they produce tend to vary across the aperture, and are often weaker at the optical axis. The rotation they produce thus varies accordingly.
Therefore, the need exists for generally more compact optical isolators that produce the desired rotation at desired wavelengths with uniform magnetic fields while avoiding the problems presented by increasing the rod length, changing optical rod materials, or using an unwieldy number of frustratingly small magnets.